Crystallizable sealing glass compositions in powdered form have been used to seal television picture tubes to provide devitrified glass seals. The resultant seals must have a suitable combination of properties such as appropriate thermal expansion characteristics, good flow to produce proper fillets, good wetting to provide strong adhesive characteristics to the glass parts being sealed, and good crystallization properties, including a proper rate of crystallization and proper size and distribution of crystals, to allow formation of strong crystallized seal within a reasonable thermal soak time.
Generally, in the past, in an attempt to produce desireable crystals, the glasses were self-nucleating through chemical makeup, or they were nucleated by adding pre-crystallized glass, or by adding various simple refractory silicates. These attempts were not successful in obtaining a sealing glass with a wide sealing temperature tolerance (sealing at 440.degree. C. to 460.degree. C.), and these glasses did not have the optimum crystal development to produce strong adhesion.
The use of precrystallized glass as a nucleating agent is described in the Hudecek U.S. Pat. No. 3,947,279, assigned to Owens-Illinois, Inc. This patent is incorporated by reference for its background content and its description of standard tests in the art such as the D.T.A. (Differential Thermal Analysis) curve and the button flow test.
U.S. Pat. No. 3,963,505 to Dumesnil (Technology Glass Corporation) shows crystalizable lead boron glasses or lead zinc boron glasses in powdered form, the glasses being blended with relatively large amount of a non-inert zinc oxide containing material that includes zinc zirconium silicate. The patent is seeking a lower sealing temperature and uses the zinc oxide containing material as a filler in relatively large amounts, at least 3 wt. % and as high as 30 wt. %. The filler acts as a source of zinc oxide, resulting in a glass which has lower sealing temperature and faster crystallization. This type of glass is suitable for sealing microelectronic packages, but is not suitable for sealing television picture tubes.
There is a need to provide a crystallizable sealing glass which can be processed at faster rates (at higher temperatures) in television bulb sealing and a glass that has a broader tolerance for sealing temperature variation, say from 440.degree. C. to 460.degree. C.